Circuit materials with improved bond, method of manufacture thereof, and articles formed therefrom

ABSTRACT

A circuit material, comprising a conductive metal layer or a dielectric circuit substrate layer and an adhesive layer disposed on the conductive metal layer or the dielectric substrate layer, wherein the adhesive comprises a poly(arylene ether) and a polybutadiene or polyisoprene polymer.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefits of U.S. Provisional PatentApplication Ser. No. 60/821,710 filed Aug. 8, 2006 and U.S.Non-Provisional patent application Ser. No. 11/829,406 filed Jul. 27,2007, which applications are fully incorporated herein by reference.

BACKGROUND

This invention generally relates to circuit materials, methods ofmanufacture of the circuit materials, and articles formed therefrom.

As used herein, a circuit material is an article used in the manufactureof circuits and multi-layer circuits, and includes circuit laminates,bond plies, resin coated conductive layers, and cover films. A circuitlaminate is a type of circuit material that has a conductive layer,e.g., copper, fixedly attached to a dielectric substrate layer. Doubleclad circuit laminates have two conductive layers, one on each side ofthe dielectric substrate layer. Patterning a conductive layer of alaminate, for example by etching, provides a circuit. Multilayercircuits comprise a plurality of conductive layers, at least one ofwhich contains a conductive wiring pattern. Typically, multilayercircuits are formed by laminating one or more circuits together usingbond plies and, in some cases, resin coated conductive layers, in properalignment using heat and/or pressure. After lamination to form themultilayer circuit, known hole-forming and plating technologies can beused to produce useful electrical pathways between conductive layers.

Historically, circuit substrates have been made with glassfabric-reinforced epoxy resins. The relatively polar epoxy materialbonds comparatively well to metallic surfaces such as copper foil.However, the polar groups in the epoxy resin also lead to a relativelyhigh dielectric constant and high dissipation factor. Electronic devicesthat operate at higher frequencies require use of circuit substrateswith low dielectric constants and low dissipation factors. Betterelectrical performance is achieved by using comparatively nonpolar resinsystems, such as those based on polybutadiene, polyisoprene, orpolyphenylene oxide polymer systems. An unwanted consequence of thelower polarity of these resins systems is an inherently lower bond tometallic surfaces.

In addition, as electronic devices and the features thereon becomesmaller, manufacture of dense circuit layouts is facilitated by use ofsubstrates with a high glass transition temperature. However, whendielectric substrates with low dielectric constants, low dissipationfactors, and high glass transition temperatures are used, adhesionbetween the conductive layer and the dielectric substrate layer can bereduced. Adhesion can be even more severely reduced when the conductivelayer is a low or very low roughness copper foil (low profile copperfoil). Such foils are desirably used in dense circuit designs to improvethe etch definition and in high frequency applications to lower theconductor loss due to roughness.

A number of efforts have been made to improve the bonding betweendielectric circuit substrates and the conductive layer surface. Forexample, use of various specific polymeric compositions has beendisclosed. PCT Application No. 99/57949 to Holman discloses using anepoxy or phenoxy resin having a molecular weight greater than about4,500 to improve the peel strength of a circuit laminate. U.S. Pat. No.6,132,851 to Poutasse also discloses use of a phenolic resoleresin/epoxy resin composition-coated metal foil as a means to improveadhesion to circuit substrates. U.S. Pat. No. 4,954,185 to Kohmdescribes a two-step process for producing a coated metal foil forprinted circuit board laminates, the first being a chemical process tocreate a metal oxide layer on the metal substrate surface, and thesecond being the application of a poly(vinyl acetal)/thermosettingphenolic composition. Gardeski, in U.S. Pat. No. 5,194,307, describes anadhesive composition having one or more epoxy components and a highmolecular weight polyester component. The cured adhesive layer isflexible and can be used for bonding metal foil to flexible circuitsubstrate (e.g., polyimide film). Yokono et al. describe improvedadhesion in a copper clad circuit laminate in U.S. Pat. No. 5,569,545,obtained by use of various sulfur-containing compounds that presumablycrosslink with the resin and chemically bond to the copper. The presenceof sulfur-containing compounds can be undesirable, giving rise to anincreased tendency to corrode. U.S. Patent Publication No. 2005/0208278to Landi et al. discloses the use of an adhesion-promoting elastomericlayer comprising a non-sulfur curing agent. However, in practice it hasbeen found that the elastomeric adhesion promoting layers can result ina soft surface, increasing the possibility of handling damage duringprocessing. Finally, Poutasse and Kovacs, in U.S. Pat. No. 5,622,782,use a multi-component organosilane layer to improve foil adhesion withanother substrate. Copper foil manufacturers can apply a silanetreatment to their foils as the final production step, and the silanecomposition, which is often proprietary, is commonly selected to becompatible with the substrate of the customer.

As noted by Poutasse et al. in U.S. Pat. No. 5,629,098, adhesives thatprovide good adhesion to metal and substrate (as measured by peelstrength) generally have less than satisfactory high temperaturestability (as measured in a solder blister resistance test). Conversely,adhesives that provide good high temperature stability generally haveless than satisfactory adhesion. There accordingly remains a need in theart for methods for improving the bond between a conductive metal and acircuit substrate, particularly thin, rigid, thermosetting substrateshaving low dielectric constants, dissipation factors, and high glasstransition temperatures, that maintains adhesiveness at hightemperatures. It would be advantageous if the adhesive did not requireB-staging, and/or that use of the adhesive did not adversely affect theelectrical and mechanical properties of the resulting circuit materials.

SUMMARY OF INVENTION

In one embodiment, an adhesive useful for forming a circuit laminatecomprises a poly(arylene ether), preferably a carboxy-functionalizedpoly(arylene ether), and a co-curable polybutadiene or polyisoprenepolymer, preferably a carboxy-functionalized polybutadiene orpolyisoprene polymer. In another embodiment, the adhesive furthercomprises an elastomeric block copolymer comprising units derived froman alkenyl aromatic compound and a conjugated diene.

In another embodiment, a circuit material for forming a circuit laminatecomprises a conductive layer or a dielectric circuit substrate; and anadhesive layer disposed on at least a portion of a surface of theconductive layer or the dielectric circuit substrate, wherein theadhesive layer comprises a poly(arylene ether), preferably acarboxy-functionalized poly(arylene ether), and a co-curablepolybutadiene or polyisoprene polymer, preferably acarboxy-functionalized polybutadiene or polyisoprene polymer. In stillanother embodiment, the adhesive layer further comprises an elastomericblock copolymer comprising units derived from an alkenyl aromaticcompound and a conjugated diene. The cured dielectric circuit substrateand the cured adhesive layer can have a dielectric constant of less thanabout 3.8 and a dissipation factor of less than about 0.007, eachmeasured at 10 gigahertz.

In another embodiment, a circuit laminate comprises one of theabove-described adhesive layers disposed between a conductive layer anda circuit substrate. In still another embodiment, the adhesive layerconsists essentially of a poly(arylene ether) or a carboxy-functionalpoly(arylene ether).

A method of forming a low dielectric constant, low dissipation factorcircuit laminate comprises disposing one of the above-described adhesivelayers between a conductive layer and a circuit substrate, andlaminating the layers.

In yet another embodiment, a circuit comprises the above-describedcircuit materials and/or laminates.

In still another embodiment, a multi-layer circuit comprises theabove-described circuit materials and/or circuit laminates.

The invention is further illustrated by the following drawings, detaileddescription, and examples.

BRIEF DESCRIPTION OF DRAWINGS

Referring now to the exemplary drawings wherein like elements arenumbered alike in the several figures:

FIG. 1 shows an exemplary circuit material with an adhesive layerdisposed on a conductive layer, e.g., a copper foil.

FIG. 2 shows an exemplary circuit material with an adhesive layerdisposed on a dielectric substrate layer.

FIG. 3 shows an exemplary circuit laminate comprising an adhesive layer.

FIG. 4 shows an exemplary double clad circuit laminate comprising twoadhesive layers.

FIG. 5 shows an exemplary double clad circuit comprising two adhesivelayers.

FIG. 6 shows an exemplary multi-layer circuit comprising three adhesivelayers.

DETAILED DESCRIPTION

It has unexpectedly been discovered by the inventors hereof thatrelatively nonpolar resins can be used to improve the adhesion between aconductive metal and a relatively nonpolar dielectric substratematerial. Accordingly, described herein are bond improvement adhesivecompositions comprising a poly(arylene ether); optionally, apolybutadiene or polyisoprene polymer, preferably a carboxylatedpolybutadiene or polyisoprene polymer; and optionally, an elastomericblock copolymer comprising units derived from an alkenyl aromaticcompound and a conjugated diene. The poly(arylene ether) can alsooptionally be carboxy-functionalized. The combination of thesecomponents provides enhanced adhesion between a conductive metal layerand a circuit substrate, as well as improved flame resistance. Theimproved bond strength is advantageously maintained at hightemperatures, such as those encountered during soldering operations(e.g., 550° F. or 288° C.). In a particularly advantageous feature, useof the adhesive composition does not significantly adversely affect theelectrical properties of the resultant circuit laminate, such as lowdielectric constant, low dissipation factor, low water absorption, andimproved dielectric breakdown strength.

The poly(arylene ether) can be in the form of a homopolymer or acopolymer, including a graft or a block copolymer. Combinations ofvarious forms can be used. Poly(arylene ether)s comprise a plurality ofstructural units of formula (1):

wherein for each structural unit, each R and R′ is independentlyhydrogen, halogen, primary or secondary C₁₋₇ alkyl, phenyl, C₁₋₇aminoalkyl, C₁₋₇ alkenylalkyl, C₁₋₇ alkynylalkyl, C₁₋₇ alkoxy, C₆₋₁₀aryl, and C₆₋₁₀ aryloxy. In some embodiments, each R is independentlyC₁₋₇ alkyl or phenyl, for example, C₁₋₄ alkyl, and each R′ isindependently hydrogen or methyl.

Exemplary poly(arylene ether)s include poly(2,6-dimethyl-1,4-phenyleneether), poly(2,6-diethyl-1,4-phenylene ether),poly(2,6-dipropyl-1,4-phenylene ether),poly(2-methyl-6-allyl-1,4-phenylene ether),poly(di-tert-butyl-dimethoxy-1,4-phenylene ether),poly(2,6-dichloromethyl-1,4-phenylene ether,poly(2,6-dibromomethyl-1,4-phenylene ether),poly(2,6-di(2-chloroethyl)-1,4-phenylene ether),poly(2,6-ditolyl-1,4-phenylene ether), poly(2,6-dichloro-1,4-phenyleneether), poly(2,6-diphenyl-1,4-phenylene ether), andpoly(2,5-dimethyl-1,4-phenylene ether). A useful poly(arylene ether)comprises 2,6-dimethyl-1,4-phenylene ether units, optionally incombination with 2,3,6-trimethyl-1,4-phenylene ether units.

The poly(arylene ether) can be functionalized so as to provide afunctional group that enhances adhesion between a conductive metal layerand a circuit substrate layer. Functionalization can be accomplishedusing a polyfunctional compound having in the molecule both (i) acarbon-carbon double bond or a carbon-carbon triple bond, and (ii) oneor more of a carboxy group, including a carboxylic acid, anhydride,amide, ester, or acid halide. In one embodiment the functional group isa carboxylic acid or ester group. Examples of polyfunctional compoundsthat can provide a carboxylic acid functional group include maleic acid,maleic anhydride, fumaric acid, and citric acid.

In particular, suitable functionalized poly(arylene ether)s include thereaction product of a poly(arylene ether) and a cyclic carboxylic acidanhydride. Examples of suitable cyclic anhydrides are maleic anhydride,succinic anhydride, glutaric anhydride, adipic anhydride, and phthalicanhydride, more specifically, maleic anhydride. Modified poly(aryleneethers) such as maleinized poly(arylene ethers) can be produced bymethods as described in U.S. Pat. No. 5,310,820, or are commerciallyavailable. Examples of commercially available suitable modified andunmodified poly(arylene ethers) include PPE-MA from Asahi (a maleinizedpoly(arylene ether)), OPE-Sty from Mitui Gas Chemicals (a styreneterminated poly(arylene ether)), and Blendex HPP820 from Chemtura (anunmodified poly(arylene ether)).

In some embodiments, the adhesives further comprise a polybutadiene orpolyisoprene polymer. A “polybutadiene or polyisoprene polymer” as usedherein includes homopolymers derived from butadiene, homopolymersderived from isoprene, and copolymers derived from butadiene and/orisoprene and/or less than 50 weight percent (wt %) of a monomerco-curable with the butadiene and/or isoprene. Suitable monomersco-curable with butadiene and/or isoprene include monoethylenicallyunsaturated compounds such as acrylonitrile, ethacrylonitrile,methacrylonitrile, alpha-chloroacrylonitrile, beta-chloroacrylonitrile,alpha-bromoacrylonitrile, C₁₋₆ alkyl(meth)acrylates (for example,methyl(meth)acrylate, ethyl(meth)acrylate, n-butyl(meth)acrylate,n-propyl(meth)acrylate, and isopropyl(meth)acrylate), acrylamide,methacrylamide, maleimide, N-methyl maleimide, N-ethyl maleimide,itaconic acid, (meth)acrylic acid, alkenyl aromatic compounds asdescribed below, and combinations comprising at least one of theforegoing monoethylenically unsaturated monomers.

The co-curable polybutadiene or polyisoprene polymer used in theadhesive composition can be co-curable with the poly(arylene ether). Inone embodiment, the polybutadiene or polyisoprene polymer iscarboxy-functionalized. Functionalization can be accomplished using apolyfunctional compound having in the molecule both (i) a carbon-carbondouble bond or a carbon-carbon triple bond, and (ii) one or more of acarboxy group, including a carboxylic acid, anhydride, amide, ester, oracid halide. A preferred carboxy group is a carboxylic acid or ester.Examples of polyfunctional compounds that can provide a carboxylic acidfunctional group include maleic acid, maleic anhydride, fumaric acid,and citric acid. In particular, polybutadienes adducted with maleicanhydride can be used in the adhesive composition. Suitable maleinizedpolybutadiene polymers are commercially available, for example fromSartomer under the trade names RICON 130MA8, RICON 130MA13, RICON130MA20, RICON 131MA5, RICON 131MA10, RICON 131MA17, RICON 131MA20, andRICON 156MA17. Suitable maleinized polybutadiene-styrene copolymers arecommercially available, for example, from Sartomer under the trade namesRICON 184MA6. RICON 184MA6 is a butadiene-styrene copolymer adductedwith maleic anhydride having styrene content from 17 to 27 wt % andnumber average molecular weight (Mn) of about 9,900 g/mole.

In still other embodiments, the adhesives further comprise anelastomeric polymer. The elastomeric polymer can be co-curable with thepoly(arylene ether) and/or the polybutadiene or isoprene resin. Suitableelastomers include elastomeric block copolymers comprising a block (A)derived from an alkenyl aromatic compound and a block (B) derived from aconjugated diene. The arrangement of blocks (A) and (B) includes linearand graft structures, including radial teleblock structures havingbranched chains. Examples of linear structures include diblock (A-B),triblock (A-B-A or B-A-B), tetrablock (A-B-A-B), and pentablock(A-B-A-B-A or B-A-B-A-B) structures as well as linear structurescontaining 6 or more blocks in total of A and B. Specific blockcopolymers include diblock, triblock, and tetrablock structures, andspecifically the A-B diblock and A-B-A triblock structures.

The alkenyl aromatic compound providing the block (A) is represented byformula (2):

wherein each of R² and R³ is independently hydrogen, C₁-C₅ alkyl, bromo,or chloro, and each of R⁴, R⁵, R⁶, R⁷, and R⁸ is independently hydrogen,C₁-C₁₂ alkyl, C₃-C₁₂ cycloalkyl, C₆-C₁₂ aryl, C₇-C₁₂ aralkyl, C₇-C₁₂alkaryl, C₁-C₁₂ alkoxy, C₃-C₁₂ cycloalkoxy, C₆-C₁₂ aryloxy, chloro,bromo, or hydroxy. Exemplary alkenyl aromatic compounds include styrene,3-methylstyrene, 4-methylstyrene, 3,5-diethylstyrene, 4-n-propylstyrene,alpha-methylstyrene, alpha-methyl vinyltoluene, alpha-chlorostyrene,alpha-bromostyrene, dichlorostyrene, dibromostyrene,tetra-chlorostyrene, and the like, and combinations comprising at leastone of the foregoing compounds. Styrene and/or alpha-methylstyrene areoften used.

Specific examples of the conjugated dienes used to provide block (B)include 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene),2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene, specifically1,3-butadiene and isoprene. A combination of conjugated dienes can beused. The block (B) derived from a conjugated diene is optionallypartially or fully hydrogenated.

Exemplary block copolymers comprising a block (A) derived from analkenyl aromatic compound and block (B) derived from a conjugated dieneinclude styrene-butadiene diblock copolymer (SB),styrene-butadiene-styrene triblock copolymer (SBS), styrene-isoprenediblock copolymer (SI), styrene-isoprene-styrene triblock copolymer(SIS), styrene-(ethylene-butylene)-styrene triblock copolymer (SEBS),styrene-(ethylene-propylene)-styrene triblock copolymer (SEPS), andstyrene-(ethylene-butylene) diblock copolymer (SEB). Such polymers arecommercially available, for example from Shell Chemical Corporationunder the trade names KRATON D-1101, KRATON D-1102, KRATON D-1107,KRATON D-1111, KRATON D-1116, KRATON D-1117, KRATON D-1118, KRATOND-1119, KRATON D-1122, KRATON D-1135X, KRATON D-1184, KRATON D-1144X,KRATON D-1300X, KRATON D-4141, KRATON D-4158, KRATON G1726, and KRATONG-1652. KRATON D-1118 is a solid SB-SBS copolymer. This copolymer haspolystyrene end blocks and a rubbery polybutadiene mid-block with about20% SBS triblock and about 80% SB diblock. It is a low modulus, lowcohesive strength, soft rubber.

The relative amount of the poly(arylene ether)s, the polybutadiene orpolyisoprene polymer, and the elastomeric block copolymer will depend onthe particular substrate material used, the desired properties of thecircuit materials and circuit laminates, and like considerations. It hasbeen found that use of a poly(arylene ether) provides increased bondstrength between a conductive metal layer, particularly copper, and arelatively nonpolar dielectric substrate material. This result isparticularly surprising since poly(arylene ether)s are themselvesnonpolar. Use of a polybutadiene or polyisoprene polymer furtherincreases high temperature resistance of the laminates, particularlywhen these polymers are carboxy-functionalized. Use of an elastomericblock copolymer may function to compatibilize the components of theadhesive. Determination of the appropriate quantities of each componentcan be done without undue experimentation, using the guidance providedherein.

In one embodiment, the adhesive composition comprises up to 100 wt % ofthe poly(arylene)ether, specifically the carboxy-functionalizedpoly(arylene ether). In another embodiment, the adhesive compositionconsists essentially of up to 100 wt % of the poly(arylene)ether,specifically the carboxy-functionalized poly(arylene)ether. In stillanother embodiment, the adhesive composition consists of up to 100 wt %of the poly(arylene)ether, specifically the carboxy-functionalizedpoly(arylene)ether.

The adhesive composition can alternatively comprise about 20 to about 99wt %, specifically about 30 to about 80 wt %, more specifically about 40to about 60 wt % of the poly(arylene ether), preferably thecarboxy-functionalized poly(arylene ether), and about 1 to about 80 wt%, specifically about 20 to about 70 wt %, more specifically about 40 toabout 60 wt % of the polybutadiene or polyisoprene polymer, preferablythe carboxy-functionalized polybutadiene or polyisoprene polymer, eachof the foregoing amounts being based on the total weight of the polymerportion of the adhesive composition.

In still another embodiment, the adhesive composition comprises about 20to about 98 wt %, specifically about 25 to about 75 wt %, morespecifically about 30 to about 50 wt % of the poly(arylene ether),preferably the carboxy-functionalized poly(arylene ether); about 1 toabout 79 wt %, specifically about 10 to about 60 wt %, more specificallyabout 20 to about 40 wt % of the co-curable polybutadiene orpolyisoprene polymer, preferably the co-curable carboxy-functionalizedpolybutadiene or polyisoprene polymer; and about 1 to about 79 wt %,specifically about 10 to about 60 wt %, more specifically about 20 toabout 40 wt % of the elastomeric block copolymer, each based on thetotal weight of the polymer portion of the adhesive composition.

In addition to the one or more of the polymers described above, theadhesive composition can further optionally comprise additives such ascure initiators, crosslinking agents, viscosity modifiers, couplingagents, wetting agents, flame retardants, fillers, and antioxidants. Theparticular choice of additives depends upon the nature of the conductivelayer and the circuit substrate composition and are selected so as toenhance or not substantially adversely affect adhesion between aconductive layer and a circuit substrate, dielectric constant,dissipation factor, water absorbance, flame retardance, and/or otherdesired properties of the circuit material.

Suitable fillers for use in the adhesive composition include titaniumdioxide (rutile and anatase), barium titanate, strontium titanate,silica, including fused amorphous silica, corundum, wollastonite,aramide fibers (e.g., KEVLAR™ from DuPont), fiberglass, Ba₂Ti₉O₂₀, glassspheres, quartz, boron nitride, aluminum nitride, silicon carbide,beryllia, alumina, magnesia, magnesium hydroxide, mica, talcs,nanoclays, aluminosilicates (natural and synthetic), and fumed silicondioxide (e.g., Cab-O-Sil, available from Cabot Corporation), used aloneor in combination. The fillers can be in the form of solid, porous, orhollow particles. Specific fillers include rutile titanium dioxide andamorphous silica. To improve adhesion between the fillers and polymer,the filler can be treated with one or more coupling agents, such assilanes, zirconates, or titanates. Fillers, when used, are typicallypresent in an amount of about 0.05 to about 10 wt %, specifically about0.1 to about 8 wt %, based on the total weight of the adhesivecomposition.

Suitable cure initiators include those useful in initiating cure(cross-linking) of the polymers, in the adhesive composition. Examplesinclude, but are not limited to, azides, peroxides, sulfur, and sulfurderivatives. Free radical initiators are especially desirable as cureinitiators. Examples of free radical initiators include peroxides,hydroperoxides, and non-peroxide initiators such as2,3-dimethyl-2,3-diphenyl butane. Examples of peroxide curing agentsinclude dicumyl peroxide, alpha,alpha-di(t-butylperoxy)-m,p-diisopropylbenzene,2,5-dimethyl-2,5-di(t-butylperoxy)hexane-3, and2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, and mixtures comprising oneor more of the foregoing cure initiators. The cure initiator, when used,is typically present in an amount of about 0.25 wt % to about 15 wt %,based on the total weight of the adhesive composition.

Crosslinking agents are reactive monomers or polymers that increase thecross-link density upon cure of the adhesive. In one embodiment, suchreactive monomers or polymers are capable of co-reacting with a polymerin the adhesive polymer and a polymer in the circuit substratecomposition. Examples of suitable reactive monomers include styrene,divinyl benzene, vinyl toluene, divinyl benzene, triallylcyanurate,diallylphthalate, and multifunctional acrylate monomers (such asSartomer compounds available from Sartomer Co.), among others, all ofwhich are commercially available. Useful amounts of crosslinking agentsare about 0.1 to about 50 wt %, based on the total weight of theadhesive composition.

Suitable antioxidants include radical scavengers and metal deactivators.A non-limiting example of a free radical scavenger ispoly[[6-(1,1,3,3-tetramethylbutyl)amino-s-triazine-2,4-dyil][(2,2,6,6,-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino]],commercially available from Ciba Chemicals under the tradenameChimmasorb 944. A non-limiting example of a metal deactivator is2,2-oxalyldiamido bis[ethyl3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] commercially availablefrom Uniroyal Chemical (Middlebury, Conn.) under the tradename NaugardXL-1. A single antioxidant or a mixture of two or more antioxidants canbe used. Antioxidants are typically present in amounts of up to about 3wt %, specifically about 0.5 to about 2.0 wt %, based on the totalweight of the adhesive composition.

Coupling agents can be present to promote the formation of orparticipate in covalent bonds connecting a metal surface or fillersurface with a polymer. Exemplary coupling agents include3-mercaptopropylmethyldimethoxy silane and 3-mercaptopropyltrimethoxysilane. Coupling agents, when present, can be present in amounts ofabout 0.1 to about 1 wt %, based on the total weight of the adhesivecomposition.

The above-described adhesive composition can be used with a dielectriccircuit substrate and a conductive layer to make circuit materials,circuit laminates, circuits, and multilayer circuits. Suitableconductive layers include a thin layer of a conductive metal such as acopper foil presently used in the formation of circuits, for example,electrodeposited copper foils. Useful copper foils typically havethicknesses of about 9 to about 180 micrometers.

The copper foil can be made either by the electrodeposition (ED) on arotating stainless steel drum from a copper sulfate bath, or by therolling of solid copper bars. Where ED copper foil is used, the initialroughness of the base foil is created in the foil plating process on the“bath side” (or matte side) of the foil. Additional roughness is createdin a secondary plating step. Where rolled foil used, roughness isimparted to the initially smooth and shiny foil by a secondary platingstep.

This mechanical roughness can result in several drawbacks. As describedin detail by Brist et al. (Gary Brist, Stephen Hall, Sidney Clouser, andTao Liang, “Non-classical conductor losses due to copper foil roughnessand treatment,” p. 26, Circuitree, May 2005) and Ogawa et al. (N. Ogawa,H. Onozeki, N. Moriike, T. Tanabe, T. Kumakura, “Profile-free foil forhigh-density packaging substrates and high-frequency applications,” p.457, Proceedings of the 2005 Electronic Components and TechnologyConference, IEEE), the roughness on a conductor surface can result in asubstantial increase in conductor loss at high frequencies, with a roughconductor causing up to twice the conductor loss of a smooth one. Ogawaalso describes the limitations to accurate circuit fabrication, mostnotably the accurate etching of fine lines and spaces that are caused byconductor roughness.

The roughness of a copper foil is generally characterized by contactprofilometry or optical interferometry. Most foil manufacturers measureroughness with a contact profilometer, due to their long history withsuch a measurement system. Most of the values cited herein were measuredusing a Veeco Instruments WYCO Optical Profiler, using the method ofwhite light interferometry. Since the roughness may exist on severaldifferent scales and will consist of many peaks and valleys with varyingdistances from a fixed reference plane, there are many different ways tonumerically characterize the surface roughness. Two frequently reportedquantities are the RMS roughness value, Rq, and the peak-to-valleyroughness, Rz, with both reported in dimensions of length.

Conventional ED copper foil made for the circuit industry has hadtreated side Rz values of 7 to 20 micrometers (um) (corresponding to Rqvalues of about 1.2 to 4 um) when measured by the WYCO Optical Profiler.Contact profilometers tend to yield lower values, due to the stylusdeforming the copper treatment as the measurement is made. The treatedside of rolled copper foil exhibits Rz values of 3.5-5.5 um(corresponding to Rq values of 0.45-0.9 um). “Reverse treated” ED foils,such as Oak-Mitsui MLS-TOC-500 can also exhibit Rq values similar tothose of rolled foils. The lower profile ED foils currently exhibit Rzvalues of 2 to 3 um. By WYCO measurement, the shiny side of rolled foilexhibits an Rz value of about 0.7 um and a corresponding Rq of about 0.1um.

More recently, other types of low profile electrodeposited foils havebeen commercially available. These include Oak Mitsui products SQ-VLP,with an Rq value measured by the WYCO of 0.7 um and MQ-VLP with a WYCORq value of 0.47 um.

Both rolled and ED foils specially treated for the circuit industry areavailable from a number of commercial manufacturers. For example, lowprofile copper foils are commercially available from Oak Mitsui underthe trade name “TOC-500” and “TOC-500-LZ.” High profile copper foils arecommercially available from Circuit Foil under the trade name “TWS.”

Suitable dielectric circuit substrates comprise low polarity, lowdielectric constant and low loss resins, including those based onthermosetting resins such as 1,2-polybutadiene, polyisoprene,poly(etherimide) (PEI), polybutadiene-polyisoprene copolymers,poly(phenylene ether) resins, and those based on allylatedpoly(phenylene ether) resins. These materials, while exhibiting thedesirable features of low dielectric constant and low loss, also exhibitlow copper peel strength. The copper peel strength of such materials canbe significantly improved by the use of the instant invention. It isalso important that the peel strength remain relatively high at elevatedtemperatures to allow for “rework,” i.e., the removal and replacement ofsoldered components on the circuit board. Combinations of low polarityresins with higher polarity resins can also be used, examples includingepoxy and poly(phenylene ether), epoxy and poly(ether imide), cyanateester and poly(phenylene ether), and 1,2-polybutadiene and polyethylene.Compositions containing polybutadiene, polyisoprene, and/or butadiene-and isoprene-containing copolymers are especially useful.

Particularly suitable circuit substrates are thermosetting compositionscomprising a thermosetting polybutadiene and/or polyisoprene resin. Asused herein, the term “thermosetting polybutadiene and/or polyisopreneresin” includes homopolymers and copolymers comprising units derivedfrom butadiene, isoprene, or mixtures thereof. Units derived from othercopolymerizable monomers can also be present in the resin, for examplein the form of grafts. Exemplary copolymerizable monomers include, butare not limited to, vinylaromatic monomers, for example substituted andunsubstituted monovinylaromatic monomers such as styrene,3-methylstyrene, 3,5-diethylstyrene, 4-n-propylstyrene,alpha-methylstyrene, alpha-methyl vinyltoluene, para-hydroxystyrene,para-methoxystyrene, alpha-chlorostyrene, alpha-bromostyrene,dichlorostyrene, dibromostyrene, tetra-chlorostyrene, and the like; andsubstituted and unsubstituted divinylaromatic monomers such asdivinylbenzene, divinyltoluene, and the like. Combinations comprising atleast one of the foregoing copolymerizable monomers can also be used.Exemplary thermosetting polybutadiene and/or polyisoprene resinsinclude, but are not limited to, butadiene homopolymers, isoprenehomopolymers, butadiene-vinylaromatic copolymers such asbutadiene-styrene, isoprene-vinylaromatic copolymers such asisoprene-styrene copolymers, and the like.

The thermosetting polybutadiene and/or polyisoprene resins can also bemodified, for example the resins can be hydroxyl-terminated,methacrylate-terminated, carboxylate-terminated resins. Post-reactedresins can be used, such as such as epoxy-, maleic anhydride-, orurethane-modified butadiene or isoprene resins. The resins can also becrosslinked, for example by divinylaromatic compounds such as divinylbenzene, e.g., a polybutadiene-styrene crosslinked with divinyl benzene.Suitable resins are broadly classified as “polybutadienes” by theirmanufacturers, for example Nippon Soda Co., Tokyo, Japan, and SartomerCompany Inc., Exton, Pa. Mixtures of resins can also be used, forexample, a mixture of a polybutadiene homopolymer and apoly(butadiene-isoprene) copolymer. Combinations comprising asyndiotactic polybutadiene can also be useful.

The thermosetting polybutadiene and/or polyisoprene resin can be liquidor solid at room temperature. Suitable liquid resins can have a numberaverage molecular weight greater than about 5,000 but generally have anumber average molecular weight of less than about 5,000 (mostpreferably about 1,000 to about 3,000). Thermosetting polybutadieneand/or polyisoprene resins having at least 90 wt % 1, 2 addition arepreferred because they exhibit the greatest crosslink density upon cure,due to the large number of pendent vinyl groups available forcrosslinking.

The polybutadiene and/or polyisoprene resin is present in the resinsystem in an amount of up to 100 wt %, specifically about 60 wt % withrespect to the total resin system, more specifically about 10 to about55 wt %, even more specifically about 15 to about 45 wt %, based on thetotal resin system.

Other polymers that can co-cure with the thermosetting polybutadieneand/or polyisoprene resins can be added for specific property orprocessing modifications. For example, in order to improve the stabilityof the dielectric strength and mechanical properties of the electricalsubstrate material over time, a lower molecular weight ethylenepropylene elastomer can be used in the resin systems. An ethylenepropylene elastomer as used herein is a copolymer, terpolymer, or otherpolymer comprising primarily ethylene and propylene. Ethylene propyleneelastomers can be further classified as EPM copolymers (i.e., copolymersof ethylene and propylene monomers) or EPDM terpolymers (i.e.,terpolymers of ethylene, propylene, and diene monomers). Ethylenepropylene diene terpolymer rubbers, in particular, have saturated mainchains, with unsaturation available off the main chain for facilecross-linking Liquid ethylene propylene diene terpolymer rubbers, inwhich the diene is dicyclopentadiene, are preferred.

Useful molecular weights of the ethylene propylene rubbers are less than10,000 viscosity average molecular weight. Suitable ethylene propylenerubbers include an ethylene propylene rubber having a viscosity averagemolecular weight (MV) of about 7,200, which is available from UniroyalChemical Co., Middlebury, Conn., under the trade name Trilene CP80; aliquid ethylene propylene dicyclopentadiene terpolymer rubbers having amolecular weight of about 7,000, which is available from UniroyalChemical Co. under the trade name of Trilene 65; and a liquid ethylenepropylene ethylidene norbornene terpolymer, having a molecular weight ofabout 7,500, which is available from Uniroyal Chemical Co. under thename Trilene 67.

The ethylene propylene rubber is preferably present in an amounteffective to maintain the stability of the properties of the substratematerial over time, in particular the dielectric strength and mechanicalproperties. Typically, such amounts are up to about 20 wt % with respectto the total weight of the resin system, more specifically about 4 toabout 20 wt %, even more specifically about 6 to about 12 wt %.

Another type of co-curable polymer is an unsaturated polybutadiene- orpolyisoprene-containing elastomer. This component can be a random orblock copolymer of primarily 1,3-addition butadiene or isoprene with anethylenically unsaturated monomer, for example a vinylaromatic compoundsuch as styrene or alpha-methyl styrene, an acrylate or methacrylatesuch a methyl methacrylate, or acrylonitrile. The elastomer ispreferably a solid, thermoplastic elastomer comprising a linear orgraft-type block copolymer having a polybutadiene or polyisoprene block,and a thermoplastic block that preferably is derived from amonovinylaromatic monomer such as styrene or alpha-methyl styrene.Suitable block copolymers of this type include styrene-butadiene-styrenetriblock copolymers, for example those available from Dexco Polymers,Houston, Tex., under the trade name Vector 8508M, from EnichemElastomers America, Houston, Tex., under the trade name Sol-T-6302, andthose from Fina Oil and Chemical Company, Dallas, Tex., under the tradename Finaprene 401; styrene-butadiene diblock copolymers; and mixedtriblock and diblock copolymers containing styrene and butadiene, forexample those available from Shell Chemical Corporation, Houston, Tex.,under the trade name Kraton D1118. Kraton D1118 is a mixeddiblock/triblock styrene and butadiene containing copolymer, containing30 volume % styrene.

The optional polybutadiene- or polyisoprene-containing elastomer canfurther comprise a second block copolymer similar to that describedabove, except that the polybutadiene or polyisoprene block ishydrogenated, thereby forming a polyethylene block (in the case ofpolybutadiene) or an ethylene-propylene copolymer block (in the case ofpolyisoprene). When used in conjunction with the above-describedcopolymer, materials with greater toughness can be produced. Anexemplary second block copolymer of this type is Kraton GX1855(commercially available from Shell Chemical Corp.), which is believed tobe a mixture of a styrene-high 1,2-butadiene-styrene block copolymer anda styrene-(ethylene-propylene)-styrene block copolymer.

Typically, the unsaturated polybutadiene- or polyisoprene-containingelastomer component is present in the resin system in an amount of about10 to about 60 wt % with respect to the total resin system, morespecifically about 20 to about 50 wt %, or even more specifically about25 to about 40 wt %.

Still other co-curable polymers that can be added for specific propertyor processing modifications include, but are not limited to,homopolymers or copolymers of ethylene such as polyethylene and ethyleneoxide copolymers; natural rubber; norbornene polymers such aspolydicyclopentadiene; hydrogenated styrene-isoprene-styrene copolymersand butadiene-acrylonitrile copolymers; unsaturated polyesters; and thelike. Levels of these copolymers are generally less than 50 vol. % ofthe total resin system.

Free radical-curable monomers can also be added for specific property orprocessing modifications, for example to increase the crosslink densityof the resin system after cure. Exemplary monomers that can be suitablecrosslinking agents include, for example, di, tri-, or higherethylenically unsaturated monomers such as divinyl benzene, triallylcyanurate, diallyl phthalate, and multifunctional acrylate monomers(e.g., Sartomer resins available from Arco Specialty Chemicals Co.,Newtown Square, Pa.), or combinations thereof, all of which arecommercially available. The crosslinking agent, when used, is present inthe resin system in an amount of up to about 20 vol. %, specifically 1to 15 vol. %, based on the total weight of the resin.

A curing agent can be added to the resin system to accelerate the curingreaction of the polyenes having olefinic reactive sites. Specificallyuseful curing agents are organic peroxides such as, dicumyl peroxide,t-butyl perbenzoate, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane,α,α-di-bis(t-butyl peroxy)diisopropylbenzene, and2,5-dimethyl-2,5-di(t-butyl peroxy)hexyne-3, all of which arecommercially available. They can be used alone or in combination.Typical amounts of curing agent are from about 1.5 to about 10 wt % ofthe total resin composition.

The circuit substrate materials can optionally include particulatefillers. Examples of suitable fillers include titanium dioxide (rutileand anatase), barium titanate, strontium titanate, silica (particles andhollow spheres) including fused amorphous silica; corundum,wollastonite, aramide fibers (e.g., Kevlar), fiberglass, Ba₂Ti₉O₂₀,glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide,beryllia, alumina, magnesia, mica, talcs, nanoclays, aluminosilicates(natural and synthetic), and magnesium hydroxide. Combinations offillers can also be used. More specifically, rutile titanium dioxide andamorphous silica are especially desirable because these fillers have ahigh and low dielectric constant, respectively, thereby permitting abroad range of dielectric constants combined with a low dissipationfactor to be achieved in the final cured product by adjusting therespective amounts of the two fillers in the composition. These fillerscan be used alone or in combination.

The circuit substrate can optionally further include woven, thermallystable webs of a suitable fiber, specifically glass (E, S, and D glass)or high temperature polyester fibers (e.g., KODEL from Eastman Kodak).Such thermally stable fiber reinforcement provides a circuit laminatewith a means of controlling shrinkage upon cure within the plane of thelaminate. In addition, the use of the woven web reinforcement renders acircuit substrate with a relatively high mechanical strength.

Examples of the woven fiberglass web are set forth in the followingTable 1.

TABLE 1 Manufacturer Style Thickness, in. (um) Fiber Glast 519-A 0.0015(38.1) Clark-Schwebel 112 0.0032 (81.3) Clark-Schwebel 1080 0.0025(63.5) Burlington 106 0.0015 (38.1) Burlington 7628 0.0068 (172.7)

The circuit substrates can optionally include fire retardant additives,such as bromine containing flame retardants. Suitable brominated flameretardants are commercially available from, for examples AlbemarleCorporation under the trade names SAYTEX BT 93W (ethylenebistetrabromophthalimide), SAYTEX 120 (tetradecabromodiphenoxybenzene),and SAYTEX 102E (decabromodiphenoxyl oxide).

In practice, the adhesive composition can be directly applied to aconductive layer or a dielectric substrate layer as a coating (if ofsufficiently low viscosity), or dissolved or suspended, i.e., in theform of a solution. Where a solution is used, the adhesive compositionis dissolved in a suitable solvent before application. The solvent ischosen so as to dissolve the adhesive composition and to have aconvenient evaporation rate for applying and drying the coating. Anon-exclusive list of possible solvents is xylene, toluene, methyl ethylketone, methyl isobutyl ketone, hexane, and higher liquid linearalkanes, such as heptane, octane, nonane, and the like, cyclohexane,isophorone, and various terpene-based solvents. Specifically, suitablesolvents include xylene, toluene, methyl ethyl ketone, methyl isobutylketone, and hexane, and more specifically xylene and toluene. Theconcentration of the adhesive composition in solution is not criticaland will depend on the solubility of the adhesive components, the methodof application, and other factors. In general, the solution comprises 1to about 50 wt %, more specifically about 5 to about 20 wt % of theadhesive composition, based on the total weight of the adhesivesolution.

The adhesive or adhesive solution can be applied to a surface of aconductive layer or a dielectric circuit substrate material (e.g., aprepreg or a B-staged material) by known methods in the art, for exampleby dip, spray, wash, or other suitable coating technique. If theadhesive solution exhibits phase separation during coating or drying,the uniformity may be improved by increasing the solution temperature.Where a solvent is present, the adhesive solution is allowed to dryunder ambient conditions, or by forced or heated air, to form anadhesion promoting layer. Typically, the adhesion promoting layer isapplied to provide a coating weight of about 2 grams per square meter(g/m² or “gsm”) to about 15 g/m², specifically about 3 g/m² to about 8g/m². The adhesion promoting layer can be uncured or partially cured inthe drying process, or the adhesion promoting layer can be partially orfully cured, if desired, after drying.

After application of the adhesive coating, the coated conductive layeror coated circuit substrate can be stored or used directly to form acircuit laminate. The laminate can be formed by means known in the art.In one embodiment, the lamination process entails placing one or morelayers of coated or uncoated circuit substrate between one or two sheetsof coated or uncoated conductive layers (provided that an adhesive layeris disposed between at least one conductive layer and at least onedielectric substrate layer). The layered material can then be storedprior to lamination and cure, partially cured and then stored, orlaminated and cured after stacking. Lamination and curing can be by aone-step process, for example using a vacuum press, or by amultiple-step process. In an exemplary multiple-step process, aconventional peroxide cure step at temperatures of about 150° C. toabout 200° C. is conducted, and the partially cured stack can then besubjected to a high-energy electron beam irradiation cure (E-beam cure)or a high temperature cure step under an inert atmosphere. Use of atwo-stage cure can impart an unusually high degree of cross-linking tothe resulting laminate. The temperature used in the second stage istypically about 250° C. to about 400° C., or the decompositiontemperature of the resin. This high temperature cure can be carried outin an oven but can also be performed in a press, namely as acontinuation of the initial lamination and cure step. Particularlamination temperatures and pressures will depend upon the particularadhesive composition and the substrate composition, and are readilyascertainable by one of ordinary skill in the art without undueexperimentation.

In accordance with various specific embodiments, FIG. 1 shows anexemplary circuit material 10 comprising adhesive layer 14 disposed on aconductive layer, e.g., a copper foil 12. As used herein, “disposed”means at least partial intimate contact between conductive layer copperfoil and the adhesive. It is to be understood that in all of theembodiments described herein, the various layers can fully or partiallycover each other, and additional copper foil layers, patterned circuitlayers, and dielectric layers can also be present. Adhesive layer 14 canbe uncured or partially cured.

FIG. 2 shows an exemplary circuit material 20 comprising an adhesivelayer 24 disposed on a dielectric circuit substrate 22. Adhesive layer24 can be uncured or partially cured, and substrate 22 can be uncured,partially cured, or fully cured.

FIG. 3 shows an exemplary circuit laminate 30 comprising an adhesivelayer 34 disposed between a dielectric circuit substrate 32 and aconductive layer 36, e.g., a copper foil. Adhesive layer 34 can beuncured or partially cured, and substrate 32 can be uncured, partiallycured, or fully cured.

FIG. 4 shows an exemplary double clad circuit laminate 40 comprising afirst adhesive layer 42 disposed between a first conductive layer 44 anda first side of a dielectric circuit substrate 45. Second adhesive layer46 is disposed between second conductive layer 48 and a second side ofcircuit substrate 45. The first and second adhesive layers 42, 46 cancomprise the same or different polymer composition, and first and secondconductive layers 44, 48 can comprise the same or different types ofconductive layer, e.g. copper foil. It is also possible to use only oneof the adhesive layers 42, 46, or to substitute one of adhesive layers42, 43 with a bond ply as is known in the art (not shown).

FIG. 5 shows an exemplary double clad circuit 50 comprising a firstadhesive layer 52 disposed between a first conductive layer 54 and afirst side of a dielectric circuit substrate 55. Second adhesive layer56 is disposed between a patterned (e.g., etched) circuit layer 58 and asecond side of dielectric circuit substrate 55. The first and secondadhesive layers 52, 56 can comprise the same or different polymercomposition. It is also possible to use only one of the adhesive layers52, 56, or to substitute one of adhesion layers 52, 56 with a bond plyas is known in the art (not shown).

FIG. 6 shows an exemplary circuit 60 comprising the circuit material 50as described in FIG. 5. A bond ply 62 can be disposed on the side of thepatterned circuit 58 opposite adhesive layer 56, and resin-coatedconductive layer comprising a copper foil 64 disposed on bond ply 62 isdisposed on a side opposite patterned circuit 58. Optionally, and asshown in FIG. 6, a third adhesive layer 66 is disposed between bond ply62 and copper foil 64. The first, second, and third adhesive layers 52,56, 62, can comprise the same or different polymer composition, andfirst and second conductive layers 54, 64 can comprise the same ordifferent types of, e.g., copper foil.

The above-described compositions and methods provide a circuit laminatewith excellent properties. In one embodiment, the circuit laminate has adielectric constant of less than about 3.8 measured at 10 gigahertz. Inanother embodiment, the resultant circuit laminate has a dissipationfactor of less than about 0.007 measured at 10 gigahertz. In yet anotherembodiment, the circuit laminate has a dielectric constant of less thanabout 3.8 and a dissipation factor of less than about 0.007 measured at10 gigahertz. Specifically, the dielectric constant and dissipationfactor of the circuit material are within about 25%, and morespecifically within about 10% of the corresponding values for thecircuit material without the adhesive composition. In addition, it isfurther desirable that other physical properties such as dielectricbreakdown strength and water absorption are similar to and/or compatiblewith the electrical characteristics of the circuit material,specifically within about 25%, and more specifically within about 10% ofthe corresponding values for the circuit material without the adhesivecomposition.

In addition, the circuit laminate has improved bond strength. In aparticularly advantageous feature, the improved bond strength isretained at elevated temperatures. This improvement is obtained whilemaintaining (i.e., not significantly adversely affecting) the dielectricproperties of the dielectric circuit substrate material alone. Use of anadhesive as described above typically results an increased peel strengthof about 1.0, specifically about 1.5 pound per linear inch (“pli”) on½-ounce/ft² copper, over the peel strength without the adhesive. In aspecific embodiment, the circuit laminate further retains bond afterrepeated solder exposures, does not blister after solder immersion, andmaintains bond strength at elevated temperatures (up to 288° C.).

The invention is further illustrated by the following non-limitingExamples.

EXAMPLES

The materials listed in Table 2 were used in the following examples.

TABLE 2 Material name Chemical name Supplier PPE-MA Maleinizedpolyphenylene ether Asahi Blendex HPP820 Unmodified polyphenylene etherChemtura Ricon ® 184MA6 Butadiene-styrene copolymer adducted withSartomer maleic anhydride Hycar 2000 X168 Vinyl-terminated polybutadieneNoveon TE 2000 Polybutadiene with urethane linkages Nippon Soda B3000Vinyl-terminated polybutadiene Nippon Soda BN1015 Maleinatedpolybutadiene Nippon Soda Kraton ® D-1118 SB diblock copolymer (20%) andSBS triblock Shell Chemical copolymer (80%) RO-4350B Flame retardantthermosetting hydrocarbon-based Rogers Corp. circuit substrate materialRO-4233 Thermosetting hydrocarbon-based circuit Rogers Corp. substratematerial FR Flame retardant nonpolar circuit substrate materialcontaining Mg(OH)₂ (U.S. Pat. No. 7,022,404 to Sethumadhavan et al.)TOC-500 Low profile standard copper, zinc free Oak-Mitsui TOC-500-LZ Lowprofile copper, with light zinc flush Oak-Mitsui TWS High profile copperfoil Circuit Foil

Copper peel strength was tested in accordance with the “Peel strength ofmetallic clad laminates” test method (IPC-TM-650 2.4.8).

The laminates were tested for solder float by floating them on a pot ofmolten solder at a temperature of 288° C. for 10 seconds. This procedureis repeated five times on each sample. A failure in the solder floattest is noted if there is blistering or delamination of the copper foilfrom the laminate surface.

Examples 1-4 and Comparative Examples A-D

Circuit laminates were prepared using an adhesive composition as setforth in Table 3 disposed between a dielectric circuit substrate and acopper foil. The adhesive compositions in Examples 1, 1A-4 and A-Dcontained 100 parts by weight of a maleinized poly(arylene ether) (10 wt% solution in a solvent having 98% toluene and 2% xylene), 0.5 parts byweight of Varox (a peroxide cure initiator), and the indicated amountsof a functionalized polybutadiene polymer and an elastomeric blockcopolymer.

The adhesive was coated onto ½ oz./ft² TWS copper foil with an RMSsurface roughness of greater than 2 um, as measured by the WYCOinterferometer, and dried to provide a coating having dry coating basisweight of 5-6 g/m². Prepreg sheets of two different dielectricsubstrates were laminated to the treated copper foil using a press cycleconsisting of a rapid ramp to 345° F. (174° C.) and 15 minute hold at345° F. (174° C.) and then a ramp to 475° F. (246° C.) and an additionalhour hold at 475° F. (246° C.). A pressure of 1000 psi (70.3kilogram/centimeters²) is maintained throughout the cycle.

The samples were tested for solder float and if they passed, they weresubsequently tested for peel strength. Results are shown in Table 3.

TABLE 3 Component Ex. 1 Ex. 1A Ex. 2 Ex. 3 Ex. 4 Ex. A Ex. B Ex. C Ex. DPPE-MA solution 100 100 100 100 100 100 100 100 100 KRATON D- 7.5 7.57.5 7.5 12.0 7.5 7.5 7.5 — 1118 B3000 — 7.5 — — — — — — — RICON 184MA63.5 — 7.5 10.0 7.5 — — — — Hycar 2000 X168 — — — — — — 7.5 — — TE 2000 —— — — — — — 7.5 — Solder Float, Marginal Marginal Pass Pass Pass FailFail Fail Fail 288° C. (pass/fail) Bond with FR NT NT 5.3 4.0 5.6 NT NTNT NT substrate (pli) Bond with RO NT 4.51 5.4 NT NT NT NT NT NT 4350B(pli) NT—not tested.

These examples demonstrate that an adhesive in accordance with thepresent invention increases copper peel strength of a circuit laminatehaving a comparatively high profile foil. They also demonstrate theefficacy of the coating in improving adhesion to substrate compositionsother than RO4350B high frequency laminate. Furthermore, theydemonstrate the importance of the maleinized polybutadiene in improvingthe high temperature resistance of the adhesive coating.

Comparative Examples A and D show that all three components of theadhesive composition are required to simultaneously increase the copperpeel strength and pass the high temperature solder float test. Inparticular, absence of the elastomeric block copolymer (Ex. D) and/orthe carboxylated polybutadiene polymer (Exs. A-D) result in failure insolder float test. Substitution of a vinyl-terminated butadiene polymer(Ex. B) or a urethane-functionalized butadiene polymer (Ex. C) alsoresults in failure in solder float test.

Examples 1-4 show that lower amounts of carboxylated butadiene polymer(Ex. 1) result in only marginal solder behavior, but that as the contentof carboxylated butadiene polymer is increased, both satisfactory solderbehavior and improved bonding were obtained (Exs. 2 and 4). Furtherincrease in the carboxylated butadiene polymer (7.5 parts in Ex. 2 to10.0 parts in Ex. 3) resulted in a slight decrease in copper peelstrength (5.3 pli in Ex. 2 vs. 4.0 pli in Ex. 3). These examplesdemonstrate the utility of this coating to materials other than RO4350Bcircuit substrate.

Examples 2-4 were laminated to a halogen-free flame retardantMg(OH)₂-filled material described in U.S. Pat. No. 7,022,404 toSethumadhavan et al. Examples 2-4 demonstrate the utility of thiscoating with materials other than RO4350B circuit substrate. As acomparison, absence of any adhesive yields a bond strength of only 1.9pli with the halogen-free system and about 3.5 pli with RO4350B circuitsubstrate.

The adhesive composition of Example 2 was also coated on two types ofcomparatively low profile electrodeposited copper foil (Oak MitsuiSQ-VLP and TQ-VLP) and laminated to an RO4350B prepreg as describedabove. As shown in the table below, the bond of both types of foil wassubstantially increased by the use of the coating. Both samples alsopassed the 288° C. solder float testing.

Copper foil Type Peel strength (no adhesive) Peel strength with AdhesiveSQ-VLP 2.2 pli 4.5 pli TQ-VLP 2.3 pli 4.2 pli

This example demonstrates that the coating is effective at improving thebond to a wide variety of copper foils.

Examples 5-6 and Comparative Example E

In Examples 5 and 6, an adhesive solution was formed using 10 parts byweight of a solution of a maleinized poly(arylene ether) (10 wt % in asolution of 98% toluene and 2% xylene), a maleinated polybutadiene (7.5parts by weight), and an elastomeric block copolymer 7.5 parts byweight). The adhesive solution was used to form a laminate prepared withan RO4350B prepreg (6 layers) and a low profile 0.5 oz copper foil (MLSTOC-500 LZ). The copper foil side having the low zinc treatment wasplaced in contact with the adhesive layer.

The materials were all laminated in a vacuum press using a rapid ramp to345° F. (174° C.) and 15 minute hold at 345° F. (174° C.), followed by aramp to 475° F. (246° C.) and an additional hour hold at 475° F. (246°C.). A pressure of 1000 psi (70.3 kilogram/centimeters²) was maintainedthroughout the cycle.

The samples were tested for bond strength, solder float, dielectricconstant, and dissipation factor. Results are shown in Table 4.

TABLE 4 Property Ex. 5 Ex. 6 Ex. E Bond (⅛-inch) (pli) 5.3 4.7 3.0Solder Float Pass Pass Pass Dielectric Constant at 10 Ghz 3.51 3.53 3.53Dissipation Factor at 10 Ghz 0.0040 0.0042 0.0042

The results in Table 4 show that the copper peel strengths achieved withthe adhesive on zinc-coated, low profile copper foils (Exs. 5 and 6)were more than 50% higher than the value without the adhesive (Ex. E).Moreover, results from Examples 5, 6, and E indicate that use ofadhesive coating did not negatively impact the high temperaturesolderability, dielectric constant, or dissipation factor of thelaminates.

Examples 8-10 and Comparative Example G

In Examples 8-10, an adhesive solution comprising 10 parts by weight ofa maleinized poly(arylene ether) (10 wt % in a solution of 98% tolueneand 2% xylene), 7.5 parts by weight of a maleinated polybutadiene, and7.5 parts by weight of an elastomeric block copolymer was used as anadhesive for a laminate prepared with an RO4233 prepreg (3 layers) and a0.4 um RMS low profile copper foil (MLS TOC-500-LZ, 0.5 oz). The copperfoil side having the zinc treatment was placed in contact with theadhesive layer. Different coating thicknesses were used for Examples8-10 (amounts shown are on a dry weight basis). In Comparative ExampleG, no adhesive formulation was used.

The samples were laminated as described above. Test results are shown inTable 5.

TABLE 5 Components Ex. 8 Ex. 9 Ex. 10 Ex. G Coating Thickness (gsm) 3.05.0 10.0 (No Coating) Bond (pli) 4.25 4.10 4.00 2.60 Solder Float PassPass Pass Pass

Examples 8-10 show that the improvement in copper peel strength is seenover a wide range of coating weights (from 3.0 gsm (Ex. 8) to 5.0 gsm(Ex. 9) and 10.0 gsm (Ex. 10)). The wide range of coating weight did notadversely affect the high temperature solder resistance.

The adhesion of the copper foil of Example 9 and Comparative Example Gwas further tested by measuring the copper pull strength. The pullstrength was measured on 0.090-inch (0.2286-centimeter) diameter pads,by soldering a copper wire to the pad and pulling the wire perpendicularto the surface of the laminate with a tensile testing machine. The pullstrength is calculated by dividing the maximum recorded force by thearea of the pad. The results for four individual pulls of each material,reported in units of psi, are shown in Table 6.

TABLE 6 Replicate 1 Replicate 2 Replicate 3 Replicate 4 Example 9 8.4910.58 9.10 7.54 Example G 5.72 5.79 7.75 6.40

The data show that pull strengths in laminates using the inventiveadhesive are much higher than those without the adhesive. These resultssuggest that adhesive coatings improved pull strengths of the coatedsamples as compared to non-coated materials.

Examples 11-16 and Comparative Examples H-Q

The following examples demonstrate that the adhesive composition isuseful for increasing the copper peel strength with a low profile foil.They also demonstrate the efficacy of the coating in improving adhesionto polybutadiene and/or polyisoprene dielectric substrates.

Accordingly, circuit laminates were prepared using an adhesivecomposition disposed between a circuit substrate and a copper foil. Theadhesive compositions are described in Table 7 below. The adhesivecompositions in Examples 11-15 and Comparative Examples J-L comprised100 parts by weight of maleinized poly(arylene ether) (10 wt % solutionin 98% toluene and 2% xylene), 0.5 parts by weight of Varox (a peroxidecure initiator), and the indicated amounts of a functionalizedpolybutadiene polymer and an elastomeric block copolymer. In Example 16,Blendex HPP820, an unmodified poly(arylene ether) commercially availablefrom Chemtura, was substituted for the maleinized poly(arylene ether).In Example N, BN1015, a maleinated polybutadiene polymer commerciallyavailable from Nippon Soda, was substituted for the RICON 184MA6.

The adhesive was coated onto ½-oz./ft² MLS TOC-500-LZ copper foil withan RMS surface roughness of about 0.4 um, as measured by the WYCOinterferometer at a final target dry basis weight of approximately 5grams/m² (gsm) using a #28 Mayer rod and allowed to air dry in a hood.In example Q, the copper foil was uncoated.

Six 0.0033-inch (0.00838-centimeter) thick prepreg sheets of the RO4350circuit substrate were laminated to the indicated dried coated coppersamples using the above-described lamination cycle to from a 0.020-inch(0.0508-centimeter) thick laminate. The laminates were tested for solderfloat. Results are shown in Table 7.

Examples 11-16 show that all of the examples containing a poly(aryleneether) are at least 40% higher in copper peel strength than the uncoatedcontrol, Example Q. This improvement in relatively smooth copperadhesion alone is a significant improvement in the utility of thesecircuit substrates. Comparative examples H, I, M, and N demonstrate thatthe presence of the maleinized poly(arylene ether) also provides anincrease in peel strength. In none of these four cases, which containthe other components of the coating formulation but do not contain amaleinized poly(arylene ether) polymer, does the copper peel strengthexceed 3.5 pli.

Comparative examples J and K demonstrate that presence of the maleinatedpolybutadiene further improves the utility of the coating by improvingthe high temperature solder resistance of the finished laminate. It canbe seen from table 7 that all examples containing the maleinized ornon-maleinized poly(arylene ether) that also contain the maleinatedpolybutadiene exhibit both the increase in copper peel strength and passthe 288° C. solder float test. Without being bound by theory, it isbelieved that the increased polarity of the maleinated polybutadienepolymer helps improve the high temperature bond of the copper foil tothe laminate by interacting more strongly with the polar surface of thefoil.

Comparative example L demonstrates the utility of the styrene-butadieneblock copolymer in providing a smooth and uniform coating when bothtypes of poly(arylene ether) and the maleinated polybutadiene arepresent. In example L, in which the Kraton 1118 was omitted, the coatingwas noted to be “grainy” or non-uniform on a macroscopic size scale. Itis hypothesized that the styrene-butadiene copolymer acts tocompatibilize, at least on a macroscopic scale, the poly(arylene ether)polymer and maleinated polybutadiene polymers.

TABLE 7 Component 11 12 13 14 15 16 H I J K L M N P Q PPE-MA 100 100 100100 100 — — — 100 100 100 — — — — solution KRATON 7.5 7.5 7.5 12.0 3.757.5 7.5 — — 7.5 — 7.5 — 7.5  — D-1118 RICON 3.5 7.5 10.0 7.5 7.5 7.5 —7.5 — — 7.5 7.5 — — — 184MA6 Blendex — — — — — 100 — — — — — — — — —HPP820 BN1015 — — — — — — — — — — — — 7.5  — — Coating Good Good GoodGood Good Good Tacky Tacky Good Good Grainy Tacky Tacky Tacky —Appearance Solder Float, Pass Pass Pass Pass Pass Pass Pass Pass FailFail Pass Pass Pass Pass Pass 288° C. (pass/fail) Copper Peel 4.70 4.754.67 4.26 4.60 4.60 3.2 2.85 5.2 — 4.88  3.45 2.70 2.75 3.0 Strength(pli)

Examples 17-23

These Examples demonstrate the suitability of these adhesiveformulations for effective and economical coating on standard productionequipment, as well as the wide range of formulations over which thecopper peel strength and high temperature resistance are improved.

Commercial scale coating trials of 25-inch (63.5-centimeter) wide rollsof copper foil were conducted on a slot die coater using theformulations shown in Table 8 (amounts shown are in grams). The sixformulations we coated at line speeds of 30 feet/minute (fpm) (9.14meter/minute) onto both Gould TWS high profile copper foil and OakMitsui MLS-TOC-500-LZ reverse-treated (low profile) copper foil (Ex.17-Ex. 22). Formulation 20 was also coated at 60 fpm, simply todemonstrate that higher speed coating was possible (Ex. 23). The coatingbasis weights ranged from 6 to 8 gsm. The samples were dried in athree-zone in-line oven with drying temperatures of 100° C., 125° C.,and 150° C. Approximately 250 linear feet (76.2 linear meters) ofuseable material was coated for each formulation and copper foil type.

Each sample of coated copper foil was laminated to six sheets of0.003-inch (0.0076-centimeter) RO4350B prepreg to form a 0.020 inches(0.0508 centimeters) thick laminate using the above press cycle, andtested.

The data for room temperature peel strength, hot oil peel strength, andsolder float testing are reported in Table 8.

TABLE 8 Component Ex. 17 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Ex. 22 Ex. 23Toluene 12540 12540 12000 12000 11500 12000 12000 Xylene 577 577 577 577525 577 577 Blendex 1039 — 1039 1039 — — 1039 PPE (Asahi) — 1039 — — 9451039 — Kraton 808 808 404 808 735 404 808 Ricon 184 MA6 1617 1617 16172079 1890 1617 2079 Varox + Toluene 52 + 100 52 + 100 50 + 100 52 + 10052 + 100 50 + 100 52 + 100 Results with MLS-TOC-500-LZ Copper FoilCopper Peel (pli) 6.3 6.0 7.2 5.9 4.9 5.1 5.2 Hot Oil Peel, 150° C.(pli) 3.2 3.2 3.4 3.4 — — 3.2 Hot Oil Peel, 200° C. (pli) 2.1 2.0 2.22.0 — — 2.0 Solder Float, 288° C., 10 s × Pass Pass Pass Pass Pass —Pass 5 (pass/fail) Coating Weight, gsm 7.8 7.4 8.4 8.1 — — 6.0 DK @ 11GHz 3.53 3.53 3.53 3.53 — — 3.51 DF @ 11 GHz 0.0042 0.0044 0.0043 0.0044— — 0.0041 Z-CTE (ppm/° C.) 45 — — — — — 43 Results with TWS Copper FoilCopper Peel (pli) 6.8 7.1 6.9 5.9 6.1 6.1 6.2 Hot Oil Peel, 150° C.(pli) 3.8 3.8 3.9 3.6 — — 3.6 Hot Oil Peel, 200° C. (pli) 2.8 2.7 3.12.5 — — 2.4 Solder Float, 288° C., 10 s × Pass Pass Pass Pass Pass —Pass 5 (pass/fail) Coating Weight (gsm) 7.5 7.7 8.1 7.8 7.4 — 6.3 DK @11 GHz 3.50 3.49 3.48 3.47 — — 3.51 DF @ 11 GHz 0.0045 0.0044 0.00430.0043 — — 0.0042

Peel strength values for the uncoated TWS copper are about 3.5 pli andthe value for the uncoated MLS-TOC-500 copper is less than 2 pli (datanot shown). The hot oil peel strength data above demonstrate sufficientbond at high temperature to allow for robust “reworkability.” The CTE,dielectric constant and loss tangent data show that the adhesive coatingdoes not have a deleterious effect on these properties.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise. The endpoints of all rangesdirected to the same characteristic or component are independentlycombinable and inclusive of the recited endpoint. All references areincorporated herein by reference. As used herein and throughout,“disposed,” “contacted,” and variants thereof refers to the complete orpartial physical contact between the respective materials, substrates,layers, films, and the like. Further, the terms “first,” “second,” andthe like herein do not denote any order, quantity, or importance, butrather are used to distinguish one element from another.

While specific embodiments have been shown and described, variousmodifications and substitutions can be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation.

What is claimed is:
 1. A circuit laminate, comprising anadhesion-improving adhesive layer disposed between a conductive copperlayer and a dielectric substrate layer without any intervening layers,wherein the composition of the adhesive layer comprises a poly(aryleneether); a polybutadiene or polyisoprene polymer comprising butadiene,isoprene, or butadiene and isoprene units, and zero to less than 50weight percent of co-curable monomer units; and an elastomeric blockcopolymer comprising units derived from an alkenyl aromatic compound anda conjugated diene; wherein the adhesive layer is present in an amountof about 2 to about 15 grams per square meter.
 2. The circuit laminateof claim 1, wherein the polybutadiene or polyisoprene polymer isobtained by co-curing the butadiene, isoprene, or butadiene and isopreneunits with styrene and/or alpha-methylstyrene units.
 3. The circuitlaminate of claim 1, wherein the polybutadiene or polyisoprene polymeris not an elastomeric block copolymer.
 4. The circuit material of claim1, wherein the poly(arylene ether) is carboxy-functionalized.
 5. Thecircuit material of claim 1, wherein the poly(arylene ether) is thereaction product of a poly(arylene ether) and a cyclic anhydride.
 6. Thecircuit material of claim 1, wherein the poly(arylene ether) is thereaction product of a poly(arylene ether) and maleic anhydride.
 7. Thecircuit material of claim 1, wherein the polybutadiene or polyisoprenepolymer is carboxy-functionalized.
 8. The circuit material of claim 7,wherein the carboxy-functionalized polybutadiene or polyisoprene polymeris the reaction product of a polybutadiene or polyisoprene polymer and acyclic anhydride.
 9. The circuit material of claim 8, wherein thecarboxy-functionalized polybutadiene or polyisoprene polymer is amaleinized polybutadiene-styrene or maleinized polyisoprene-styrenecopolymer.
 10. The circuit material of claim 1, wherein the compositionof the adhesive layer comprises about 20 to about 99 wt. % of thepoly(arylene ether) and about 1 to about 80 wt. % of the polybutadieneor polyisoprene polymer, each based on the combined weight of thepoly(arylene ether) and the polybutadiene or polyisoprene polymer. 11.The circuit material of claim 1, wherein, in the elastomeric blockcopolymer, the alkenyl aromatic compound is styrene and the conjugateddiene is polybutadiene.
 12. The circuit material of claim 11, whereinthe elastomeric block copolymer is styrene-butadiene diblock copolymer,styrene-butadiene-styrene triblock copolymer, styrene-isoprene diblockcopolymer, styrene-isoprene-styrene triblock copolymer,styrene-(ethylene-butylene)-styrene triblock copolymer,styrene-(ethylene-propylene)-styrene triblock copolymer,styrene-(ethylene-butylene) diblock copolymer, or a combinationcomprising at least one of the foregoing copolymers.
 13. The circuitmaterial of claim 12, wherein the block copolymer is a styrene-butadienediblock copolymer, styrene-butadiene-styrene triblock copolymer, or acombination comprising at least one of the foregoing copolymers.
 14. Thecircuit material of claim 13, wherein the block copolymer is acombination of styrene-butadiene diblock copolymer andstyrene-butadiene-styrene triblock copolymer.
 15. The circuit materialof claim 1, wherein the composition of the adhesive layer comprisesabout 20 to about 98 wt. % of the poly(arylene ether), about 1 to about79 wt. % of the polybutadiene or polyisoprene polymer, and about 1 toabout 79 wt. % of the elastomeric block copolymer, each based on thecombined weight of the poly(arylene ether), the polybutadiene orpolyisoprene polymer, and the elastomeric block copolymer.
 16. Thecircuit material of claim 1, wherein the adhesive layer is present in anamount of about 3 to about 8 grams per square meter.
 17. The circuitmaterial of claim 1, wherein the conductive copper layer is a copperfoil having an RMS roughness of less than 0.7 micrometers.
 18. Thecircuit laminate of claim 1, wherein the dielectric substrate layer andthe adhesive layer have a dielectric constant of less than about 3.8 anda dissipation factor of less than about 0.007, each measured atfrequencies from 1 to 10 gigahertz.
 19. The circuit laminate of claim 1,further comprising a second adhesive layer disposed between a secondcopper conductive layer and a second side of the dielectric substratelayer opposite to side of the dielectric substrate layer on which thefirst said adhesive layer is disposed.
 20. A multi-layer circuitcomprising the circuit material of claim
 1. 21. The circuit material ofclaim 1, wherein the composition of the adhesive layer comprises 20 toabout 70 wt. % of the polybutadiene or polyisoprene polymer based on thetotal weight of the polymer portion of the composition.
 22. The circuitmaterial of claim 1, wherein filler, if present in the adhesive layer,is in an amount of about 0.1 to 8 weight percent.
 23. The circuitmaterial of claim 1, wherein the presence of the adhesive layerincreases the peel strength of the conductive copper layer in thecircuit material and the adhesive layer fully covers the dielectricsubstrate layer.
 24. A circuit material, comprising a conductive copperlayer and a dielectric substrate layer; and an adhesion-improvingadhesive layer disposed between the conductive copper layer and thedielectric substrate layer without any intervening layers, wherein thecomposition of the adhesive layer comprises a poly(arylene ether); and apolybutadiene or polyisoprene polymer comprising butadiene, isoprene, orbutadiene and isoprene units, and zero to less than 50 weight percent ofco-curable monomer units; an elastomeric block copolymer comprisingunits derived from an alkenyl aromatic compound and a conjugated diene;wherein the resin composition of the dielectric substrate layercomprises a low polarity thermosetting resin and wherein the adhesivelayer is present in an amount of about 2 to about 15 grams per squaremeter.
 25. The circuit material of claim 24, wherein the adhesive layerfully covers the dielectric substrate layer.
 26. The circuit material ofclaim 24, wherein the resin composition of the dielectric substratelayer consists essentially of low polarity thermosetting resin material.27. The circuit material of claim 24, wherein the resin composition ofthe dielectric substrate layer comprises a thermosetting polybutadieneand/or polyisoprene resin homopolymer or copolymer comprising unitsderived from butadiene, isoprene, or mixtures thereof and optionallycopolymerizable monomers comprising vinylaromatic monomers and/ordivinylaromatic monomers.
 28. A circuit material, comprising aconductive copper layer and a dielectric substrate layer; and anadhesion-improving adhesive layer disposed between the conductive copperlayer and the dielectric substrate layer without any intervening layers,wherein the composition of the adhesive layer comprises a poly(aryleneether); a carboxy-functionalized polybutadiene or polyisoprene polymercomprising butadiene, isoprene, or butadiene and isoprene units, andzero to less than 50 weight percent of co-curable monomer units; and anelastomeric block copolymer comprising units derived from an alkenylaromatic compound and a conjugated diene; wherein the adhesive layer ispresent in an amount of about 2 to about 15 grams per square meter; andwherein the dielectric substrate layer comprises a low polarity, lowdielectric constant and low loss resin selected from the groupconsisting of a thermosetting polybutadiene and/or polyisoprene resinhomopolymer or copolymer comprising units derived from butadiene,isoprene, or mixtures thereof.
 29. The circuit material of claim 28,wherein the thermosetting polybutadiene and/or polyisoprene resin in thedielectric substrate layer is a liquid at room temperature.
 30. Thecircuit material of claim 29, wherein the thermosetting polybutadieneand/or polyisoprene resin is present in the resin system in an amount of15 to 100 wt. % based on the total resin system.
 31. The circuitmaterial of claim 28, wherein the dielectric substrate layer comprises athermosetting polybutadiene and/or polyisoprene resin homopolymer orcopolymer in combination with a co-curable polymer selected from thegroup consisting of ethylene propylene elastomer, unsaturatedpolybutadiene- or polyisoprene-containing elastomer, a polybutadiene- orpolyisoprene-containing elastomer in which the polybutadiene orpolyisoprene block is hydrogenated, a homopolymer or copolymer ofethylene, a natural rubber, a norbornene copolymer, a hydrogenatedstyrene-isoprene-styrene copolymer, a hydrogenatedbutadiene-acrylonitrile copolymer, and an unsaturated polyester.
 32. Thecircuit material of claim 31, wherein the dielectric substrate layerfurther comprises free radical-curable monomers to increase thecrosslink density of the resin system after cure.
 33. The circuitmaterial of claim 32, wherein the dielectric substrate layer comprises,based on the total resin system: 15 to 100 wt. % thermosettingpolybutadiene and/or polyisoprene resin homopolymer or copolymercomprising units derived from butadiene, isoprene, or mixtures thereof;0 to 20 wt. % of an ethylene propylene elastomer or 10 to 60 wt. % ofunsaturated polybutadiene and/or polyisoprene-containing elastomer; and0 to 20 vol. % of free radical-curable monomer.
 34. The circuit materialof claim 33, wherein the wherein the dielectric substrate layercomprises, based on the total resin system, of 4 to 20 wt. % of ethylenepropylene elastomer or 10 to 60 wt. % of unsaturated polybutadieneand/or polyisoprene-containing elastomer and 1 to 20 vol. % of freeradical-curable monomer.